Many conventional imaging devices, such as printers, use ribbons for transferring images onto printable media fed into the imaging devices. Typically, each of these printers has a print head to print images on the printable media, a platen to press the printable media against the print head and to move the printable media, and a ribbon mechanism to supply ribbons. The printable media, e.g., a label roll or a sheet of paper, and the ribbon are loaded between the platen and the print head. In an operational position, the platen presses the printable media and the ribbon tightly against the print head to form a contact area. Thus, when the platen rotates the printable media and the ribbon will be accordingly moved forward or backward depending on the rotational direction of the platen. The contact area defines a current printing location of the printable medium where images are to be formed thereon. After the print head prints images on the current contact area of the printable medium, the platen rotates to move the printable medium, together with the ribbon, forward toward a front side of the printer in order to position a subsequent contact area of the printable medium for printing. The platen is normally driven by a motor incorporated within the printer to control a rotational speed of the platen and, thus, to control the speed of the printable media and the ribbon moving past the print head.
All ribbons contain image forming materials, such as carbon particles, to be transferred, e.g., by thermal transfer methods, as images onto the printable media. They are often in the form of a ribbon strip wrapped to form a ribbon roll to be mounted on the ribbon mechanism of the printer. Ordinarily, only one side of the ribbon strip has the image forming materials deposited thereon. If, for any reason, the ribbon strip wrinkles during operation, the image forming materials within folds of such wrinkles cannot be properly transferred onto the printable media. Therefore, great effort has been taken by every conventional printer manufacturer to prevent wrinkles of the ribbon strip. Otherwise, if any wrinkle of the ribbon strip occurs, it will often cause errors or distortions of the images to be printed on the printable media, and perhaps a whole section of the printable media will be left blank.
Most commercially available ribbons are essentially in a form of a very thin web. Due to this thinness, it is very easy for a ribbon strip to wrinkle or to buckle when it passes through the printer. Alignment of the ribbon strip, as it passes through a ribbon path in the printer, is, therefore, critical to prevent wrinkles of the ribbon strip. As a result, most conventional printers use various techniques to align their ribbon strips when the ribbon strips pass through their respective ribbon paths. For example, some printers use ribbon edge guides to confine their ribbon strips within predetermined pathways of the printers while others use mechanisms to impart tension on the ribbon strips to prevent wrinkles. Nevertheless, these techniques are sometimes not very effective for eliminating the ribbon wrinkles that commonly occur to the ribbon strips during operation.
Part of the reason that the above-mentioned conventional techniques are not very effective in preventing wrinkles of the ribbon strips is that the ribbon strips are often over constrained or under-constrained in the printers. For example, if the ribbon strip is not perfectly aligned by the printer, uneven stress will be imparted to various portions of the ribbon strip when the ribbon strip is moved from a supply roll mounted on the ribbon mechanism toward the print head and, thereafter, toward a take-up roll. The existence of the uneven stress on the ribbon strip is due to the fact that, when the ribbon strip is unaligned or tilted, the printer will pull a first part of the ribbon strip with a stronger tension than it pulls a second part of the ribbon strip. For example, if the ribbon strip is misaligned with the platen, the platen may pull a first end of the ribbon strip with a stronger tension than it pulls a second end, opposite to the first, of the ribbon strip. Moreover, when the ribbon strip is misaligned, the first part of the ribbon strip may be moved by the printer instantaneously faster than the second part moved by the printer. The first part of the ribbon strip thus has a different tension than that of the second part, and wrinkles will almost inevitably occur in the ribbon strip due to the tension difference. Once wrinkles occur, the thin-web nature of the ribbon strip also promotes the wrinkles to further accumulate or transmit. Thus, an event that starts with a small uneven stress on the ribbon strip due to a slight misalignment may cause the ribbon strip to twist and propagate the twisting to eventually corrupt the printing.
Many conventional printers have insufficient mechanisms to resolve the above-mentioned, alignment induced, uneven stress problem. Often, they simply provide ribbon guides in the printers or apply pulling and/or dragging forces on the ribbon strips in the hope of preventing wrinkles. Furthermore, once the uneven stress occurs to the ribbon strip, a conventional printer cannot realign the ribbon strip in real time to prevent wrinkles before the ribbon strip reaches the print head. For a conventional printer, the ribbon strip can only be readjusted after the problem of wrinkles has been noted by a user. This post problem correction disrupts the printing process, wastes and/or damages printable media, takes operator time, and is a corrective, rather than a preventive, measure.